## Les Houches Accord

The Les Houches Accord (LHA) for user processes Boo01 is the standard way to input parton-level information from a matrix-elements-based generator into PYTHIA. The conventions for which information should be stored has been defined in a Fortran context, as two commonblocks. Here a C++ equivalent is defined, as a single class. The most common application is to read input from a Les Houches Event File (LHEF) Alw06, but it is also possible to have a runtime interface to another program. This page will discuss runtime interface. For LHEF handling, please refer to Les Houches Event Files.

The `LHAup` class is a base class, containing reading and printout functions, plus two pure virtual functions, one to set initialization information and one to set information on each new event. Derived classes have to provide these two virtual functions to do the actual work. The existing derived classes are for reading information from a Les Houches Event File, from the respective Fortran commonblocks, or from PYTHIA 8 itself.

You are free to write your own derived classes, using the rules and methods to be described below. Normally, pointers to objects of such derived classes should be handed in with the `Pythia::setLHAupPtr( LHAup*)` method. However, with the LHEF format a filename can replace the pointer, see further below.

Let us now describe the methods at your disposal to do the job.

the base class constructor takes the choice of mixing/weighting strategy as optional input argument, and calls `setStrategy`, see below. It also reserves some space for processes and particles.
the destructor does not need to do anything.
this method only sets the pointer that allows some information to be accessed, and is automatically called by `Pythia::init()`.

### Initialization

The `LHAup` class stores information equivalent to the `/HEPRUP/` commonblock, as required to initialize the event generation chain. The main difference is that the vector container now allows a flexible number of subprocesses to be defined. For the rest, names have been modified, since the 6-character-limit does not apply, and variables have been regrouped for clarity, but nothing fundamental is changed.

this pure virtual method has to be implemented in the derived class, to set relevant information when called. It should return false if it fails to set the info. In the no-beams extension this method need not do anything, since by default strategy 3 is chosen and the rest is set vanishing, but the method must exist.

Inside `setInit()`, such information can be set by the following methods: sets the properties of the first and second incoming beam, respectively (cf. the Fortran `IDBMUP(1), EBMUP(i), PDFGUP(i), PDFSUP(i)`, with `i` 1 or 2). These numbers can be used to tell which PDF sets were used when the hard process was generated, while the normal PDF Selection is used for the further event generation in PYTHIA.

sets the event weighting and cross section strategy. The default, provided in the class constructor, is 3, which is the natural value e.g. for an LHEF. chosen strategy (cf. `IDWTUP`; see Sjo06 section 9.9.1 for extensive comments). events come with non-negative weight, given in units of pb, with an average that converges towards the cross section of the process. PYTHIA is in charge of the event mixing, i.e. for each new try decides which process should be generated, and then decides whether is should be kept, based on a comparison with `xMax`. Accepted events therefore have unit weight. as option 1, except that cross sections can now be negative and events after unweighting have weight +-1. You can use `Info::weight()` to find the weight of the current event. A correct event mixing requires that a process that can take both signs should be split in two, one limited to positive or zero and the other to negative or zero values, with `xMax` chosen appropriately for the two. events come with non-negative weight, in unspecified units, but such that `xMax` can be used to unweight the events to unit weight. Again PYTHIA is in charge of the event mixing. The total cross section of a process is stored in `xSec`. as option 2, except that cross sections can now be negative and events after unweighting have weight +-1. As for option -1 processes with indeterminate sign should be split in two. events come with unit weight, and are thus accepted as is. The total cross section of the process is stored in `xSec`. as option 3, except that events now come with weight +-1. Unlike options -1 and -2 processes with indeterminate sign need not be split in two, unless you intend to mix with internal PYTHIA processes (see below). events come with non-negative weight, given in units of pb, with an average that converges towards the cross section of the process, like for option 1. No attempt is made to unweight the events, however, but all are generated in full, and retain their original weight. For consistency with normal PYTHIA units, the weight stored in `Info::weight()` has been converted to mb, however. as option 4, except that events now can come either with positive or negative weights.

Note 1

: if several processes have already been mixed and stored in a common event file, either LHEF or some private format, it would be problematical to read back events in a different order. Since it is then not feasible to let PYTHIA pick the next process type, strategies +-1 and +-2 would not work. Instead strategy 3 would be the recommended choice, or -3 if negative-weight events are required.

Note 2

: it is possible to switch on internally implemented processes and have PYTHIA mix these with LHA ones according to their relative cross sections for strategies +-1, +-2 and 3. It does not work for strategy -3 unless the positive and negative sectors of the cross sections are in separate subprocesses (as must always be the case for -1 and -2), since otherwise the overall mixture of PYTHIA and LHA processes will be off. Mixing is not possible for strategies +-4, since the weighting procedure is not specified by the standard. (For instance, the intention may be to have events biased towards larger pT values in some particular functional form.)
sets info on an allowed process (cf. `LPRUP, XSECUP, XERRUP, XMAXUP`). Each new call will append one more entry to the list of processes. The choice of strategy determines which quantities are mandatory: `xSec` for strategies +-2 and +-3, `xErr` never, and `xMax` for strategies +-1 and +-2.

Note

: PYTHIA does not make active use of the (optional) `xErr` values, but calculates a statistical cross section error based on the spread of event-to-event weights. This should work fine for strategy options +-1, but not for the others. Specifically, for options +-2 and +-3 the weight spread may well vanish, and anyway is likely to be an underestimate of the true error. If the author of the LHA input information does provide error information you may use that - this information is displayed at initialization. If not, then a relative error decreasing like 1/sqrt(n_acc), where n_acc is the number of accepted events, should offer a reasonable estimate.

update the `xSec` value of the `i`'th process added with `addProcess` method (i.e. `i` runs from 0 through `sizeProc() - 1`, see below).
update the `xErr` value of the `i`'th process added with `addProcess` method.
update the `xMax` value of the `i`'th process added with `addProcess` method.
set the header `key` to have value `val`. This is a wrapper function to the Info::setHeader function that should be used in any classes derived from LHAup.

Information is handed back by the following methods (that normally you would not need to touch): for the beam properties. for the strategy choice. for the number of subprocesses. for process `i` in the range `0 <= i < sizeProc()`. the sum of the cross sections and errors (the latter added quadratically). Note that cross section errors are only meaningful for strategies +-3.

prints the above initialization information. This method is automatically called from `Pythia::init()`, so would normally not need to be called directly by the user.

### Event input

The `LHAup` class also stores information equivalent to the `/HEPEUP/` commonblock, as required to hand in the next parton-level configuration for complete event generation. The main difference is that the vector container now allows a flexible number of partons to be defined. For the rest, names have been modified, since the 6-character-limit does not apply, and variables have been regrouped for clarity, but nothing fundamental is changed.

The LHA standard is based on Fortran arrays beginning with index 1, and mother information is defined accordingly. In order to be compatible with this convention, the zeroth line of the C++ particle array is kept empty, so that index 1 also here corresponds to the first particle. One small incompatibility is that the `sizePart()` method returns the full size of the particle array, including the empty zeroth line, and thus is one larger than the true number of particles (`NUP`).

this pure virtual method has to be implemented in the derived class, to set relevant information when called. For strategy options +-1 and +-2 the input `idProcess` value specifies which process that should be generated, while `idProcess` is irrelevant for strategies +-3 and +-4. The method should return false if it fails to set the info, i.e. normally that the supply of events in a file is exhausted. If so, no event is generated, and `Pythia::next()` returns false. You can then interrogate `Info::atEndOfFile()` to confirm that indeed the failure is caused in this method, and decide to break out of the event generation loop.

Inside a normal `setEvent(...)` call, information can be set by the following methods: tells which kind of process occurred, with what weight, at what scale, and which alpha_EM and alpha_strong were used (cf. `IDPRUP, XWTGUP, SCALUP, AQEDUP, AQCDUP`). This method also resets the size of the particle list, and adds the empty zeroth line, so it has to be called before the `addParticle` method below. gives the properties of the next particle handed in (cf. `IDUP, ISTUP, MOTHUP(1,..), MOTHUP(2,..), ICOLUP(1,..), ICOLUP(2,..), PUP(J,..), VTIMUP, SPINUP`; while `scale` is a new optional property, see further below) .

Information is handed back by the following methods: process number.

. Note that the weight stored in `Info::weight()` as a rule is not the same as the above `weight()`: the method here gives the value before unweighting while the one in `info` gives the one after unweighting and thus normally is 1 or -1. Only with strategy options +-3 and +-4 would the value in `info` be the same as here, except for a conversion from pb to mb for +-4.
scale and couplings at that scale.
the size of the particle array, which is one larger than the number of particles in the event, since the zeroth entry is kept empty (see above).
for particle `i` in the range `0 <= i < sizePart()`. (But again note that `i = 0` is an empty line, so the true range begins at 1.)

From the information in the event record it is possible to set the flavour and x values of the initiators

This information is returned by the methods the flavour and x values of the two initiators.

In the LHEF description Alw06 an extension to include information on the parton densities of the colliding partons is suggested. This optional further information can be set by which gives the flavours , the x and the Q scale (in GeV) at which the parton densities x*f_i(x, Q) have been evaluated. The last argument is normally `true`.

This information is returned by the methods where the first one tells whether this optional information has been set for the current event. (`setPdf(...)` must be called after the `setProcess(...)` call of the event for this to work.) Note that the flavour and x values usually but not always agree with those obtained by the same methods without `pdf` in their names, see explanation in the Event Information description.

The maximum scale for parton-shower evolution of a Les Houches event is regulated by the `TimeShower:pTmaxMatch` and `SpaceShower:pTmaxMatch` modes. If you want to guarantee that the input `scale` value is respected, as is often the case in matching/merging procedures, you should set both of these modes to 1. That only affects the hard process, while resonance decays are still processed using the resonance mass to set the upper limit. However, the optional `Beams:strictLHEFscale = on` setting restricts also resonance-decay emissions to be below the input `scale` value.

As a further non-standard feature, it is also possible to read in the separate scale values of all final particles. Such scale values could be used e.g. to restrict the maximum scale for shower evolutions for each parton separately. This reading will only be applied if the `Beams:setProductionScaleFromLHEF` switch is true (see `Beam Parameters` for details). This information is returned by the method `double LHAup::scale(int i)`. When no such information has been read from the LHEF, the scale defaults to -1.

prints the above information for the current event. In cases where the `LHAup` object is not available to the user, the `Pythia::LHAeventList()` method can be used, which is a wrapper for the above.
skip ahead `nSkip` events in the Les Houches generation sequence, without doing anything further with them. Mainly intended for debug purposes, e.g. when an event at a known location in a Les Houches Event File is causing problems. Will return false if operation fails, specifically if the end of an LHEF has been reached. The implementation in the base class simply executes `setEvent()` the requested number of times. The derived `LHAupLHEF` class (see below) only uses the `setNewEventLHEF(...)` part of its `setEvent()` method, and other derived classes could choose other shortcuts.

The LHA expects the decay of resonances to be included as part of the hard process, i.e. if unstable particles are produced in a process then their decays are also described. This includes Z^0, W^+-, H^0 and other short-lived particles in models beyond the Standard Model. Should this not be the case then PYTHIA will perform the decays of all resonances it knows how to do, in the same way as for internal processes. Note that you will be on slippery ground if you then restrict the decay of these resonances to specific allowed channels since, if this is not what was intended, you will obtain the wrong cross section and potentially the wrong mix of different event types. (Since the original intention is unknown, the cross section will not be corrected for the fraction of open channels, i.e. the procedure used for internal processes is not applied in this case.)

Even if PYTHIA can select resonance decay modes according to its internal tables, there is normally no way for it to know which decay angular correlations should exist in the simulated process. Therefore almost all decays are isotropic. The exceptions are Higgs and top decays, in the decay chains H → WW/ZZ → f fbar f' fbar' and t → b W → b f fbar, where the process-independent correlations implemented for internal processes are used. If part of the decay chain has already been set, however (e.g. H → WW/ZZ or t → b W), then decay is still isotropic.

The LHA standard only allows for one hard subcollision in an event. Further multiparton interactions are supposed to be handled by the internal MPI machinery. As a nonstandard feature, it is possible to input two hard subcollisions in the same event, to match the internal second hard process machinery. In such cases two partons are extracted from each of the two incoming hadrons. A restriction is that, unlike the single-subprocess case, it is important that the partons are input in the order that PYTHIA later would need. That is, the two subcollisions should follow each other, with instate preceding outstate. Any resonance decay chain should be put at the end, after both interactions. As illustration, consider double W production. With 1 and 2 labelling the two subcollisions, and A and B the two incoming hadron beams, the event record ordering should be in_A1 - in_B1 - W_1 - in_A2 - in_B2 - W_2 - f_1 - fbar_1 - f_2 - fbar_2, where f fbar is the fermion decay products of the respective W. A limitation is that currently only one input scale is used, that thereby limits all further partonic activity in the same way for both processes.

When transferring events through the runtime interface, it is worth noting that PYTHIA offers some settings to ensure the consistency of momenta, e.g. reshuffling of particle momenta to guarantee on-shell conditions, or matching the sum of incoming to the sum of outgoing momenta. The documentation of settings related to this can be found under the Transfer to the PYTHIA process record heading of Les Houches Event Files .

### A LHEF reader class

The LHEF standard (Alw06, But14) specifies a format where a single file packs initialization and event information. This has become the most frequently used procedure to process external parton-level events in Pythia, and is discussed in detail in the Les Houches Event Files section. Internally the file handling and reading is us handled by instance of the derived class `LHAupLHEF`.

The workhorses of the `LHAupLHEF` class are three methods found in the base class, so as to allow them to be reused in other contexts.

read in and set all required initialization information from the specified stream. With second argument true it will also read and store header information, as described above. Return false if it fails.
read in event information from the specified stream into a staging area where it can be reused by `setOldEventLHEF`.
store the event information from the staging area into the normal location. Thus a single `setNewEventLHEF` call can be followed by several `setOldEventLHEF` ones, so as to process the same configuration several times. This method currently only returns true, i.e. any errors should be caught by the preceding `setNewEventLHEF` call.

These three main methods build on a number of container classes and a generic LHEF reader class (called `Reader`) found in `LHEF3.h` and `LHEF3.cc`. The `Reader` handles all the parsing and storage necessary to adhere with But14. (A matching `Writer` class is also available; see documentation in `LHEF3.h` how it can be used.) All parsing that is not strictly part of the LHEF format (e.g. the reading of header information) is instead performed directly in the `LHAupLHEF` methods.

Some other small utility routines are:

always returns true in the base class, but in `LHAupLHEF` it returns false if the LHEF provided in the constructor is not found and opened correctly.
always returns false in the base class, but in `LHAupLHEF` it returns false if the `LHAupLHEF` instance is constructed to work on an input LHE file, while it returns true if the `LHAupLHEF` instance is constructed to use externally provided input streams instead. For the latter, the `LHAupLHEF` instance should have been constructed with the class constructor `LHAupLHEF(Info* infoPtrIn, istream* isIn, istream* isHeadIn, bool readHeadersIn, bool setScalesFromLHEFIn)`.
is used to send header information on to the `Info` class.

A few other methods, most of them derived from the base class, streamlines file opening and closing, e.g. if several LHE files are to be read consecutively, without the need for a complete reinitialization. This presupposes that the events are of the same kind, only split e.g. to limit file sizes.

close current event input file/stream and open a new one, to continue reading events of the same kind as before.
open and close a file, also gzip files, where an intermediate decompression layer is needed.
close main event file (LHEF) and, if present, separate header file.

### A runtime Fortran interface

The runtime Fortran interface requires linking to an external Fortran code. In order to avoid problems with unresolved external references when this interface is not used, the code has been put in a separate `include/Pythia8Plugins/LHAFortran.h` file, that is not included in any of the other library files. Instead it should be included in the user-supplied main program, and used to create a derived class that contains the implementation of two methods below that call the Fortran program to do its part of the job.

The `LHAupFortran` class derives from `LHAup`. It reads initialization and event information from the LHA standard Fortran commonblocks, assuming these commonblocks behave like two `extern "C" struct` named `heprup_` and `hepeup_`. (Note the final underscore, to match how the gcc compiler internally names Fortran files.)

The instantiation does not require any arguments.

The user has to supply implementations of the `fillHepRup()` and `fillHepEup()` methods, that is to do the actual calling of the external Fortran routines that fill the `HEPRUP` and `HEPEUP` commonblocks. The translation of this information to the C++ structure is provided by the existing `setInit()` and `setEvent()` code.

Up to and including version 8.125 the `LHAupFortran` class was used to construct a runtime interface to PYTHIA 6.4. This was convenient in the early days of PYTHIA 8 evolution, when this program did not yet contain hard-process generation, and the LHEF standard did not yet exist. Nowadays it is more of a bother, since a full cross-platform support leads to many possible combinations. Therefore this support has been removed, but can still be recuperated from previous code versions, in a reduced form up to version 8.176.

### A LHEF writer class

The main objective of the `LHAup` class is to feed information from an external program into PYTHIA. It can be used to export information as well, however. The main documentation of these features is found in the Les Houches Event Files section. Nevertheless, we quickly summarize the main workhorse functions here. There are four routines in the base class that should be called in sequence to build up the proper file structure.

Opens a file with the filename indicated, and writes a header plus a brief comment with date and time information.
Writes initialization information to the file above. Such information should already have been set with the methods described in the "Initialization" section above.
Writes event information to the file above. Such information should already have been set with the methods described in the "Event input" section above. This call should be repeated once for each event to be stored. By default the event information is lined up in columns. To save space, the alternative `verbose = false` only leaves a single blank between the information fields.
Writes the closing tag and closes the file. Optionally, if `updateInit = true`, this routine will reopen the file from the beginning, rewrite the same header as `openLHEF()` did, and then call `initLHEF()` again to overwrite the old information. This is especially geared towards programs, such as PYTHIA itself, where the cross section information is not available at the beginning of the run, but only is obtained by Monte Carlo integration in parallel with the event generation itself. Then the `setXSec( i, xSec)`, `setXErr( i, xSec)` and `setXMax( i, xSec)` can be used to update the relevant information before `closeLHEF` is called.

Warning:

overwriting the beginning of a file without upsetting anything is a delicate operation. It only works when the new lines require exactly as much space as the old ones did. Thus, if you add another process in between, the file will be corrupted.
Return the name of the LHE file above.